Nitrous oxide (N.sub.2 O) is not commonly considered an atmospheric pollutant and has not been considered a constituent of the gaseous pollutants collectively referred to as nitrogen oxides (NO.sub.x) which have received wide attention as pollutants harmful to the environment. However, recent studies indicate that N.sub.2 O in the Earth's atmosphere may be increasing by about 0.2% per year and that this increase appears to be caused by anthropogenic activity.
N.sub.2 O is a major stratospheric source of NO, is believed to be involved in destroying the ozone layer and is recognized to be a green-house gas. Because N.sub.2 O has an atmospheric lifetime of approximately 150 years, researchers are attempting to identify sources of the pollutant and to limit further production of the harmful gas. Recent reports such as an article by Thiemens and Trogler, Science, 251 (1991) 932 suggest that various industrial processes significantly contribute to the increased levels of N.sub.2 O found in the Earth's atmosphere.
For example, nitrous oxide is a by-product in the manufacture of monomers for producing 6,6- and 6,12-nylon. Approximately 1.24.times.10.sup.9 kg of nylon were produced in the United States in 1988, alone. Nylon polymers are typically formed by subjecting a dicarboxylic acid and a diamine to a condensation polymerization reaction. The most widely used dicarboxylic acid, adipic acid, is prepared primarily by oxidizing cyclohexane in air to form a cyclohexanol/cyclohexanone mixture followed by oxidizing such mixture with HNO.sub.3 to form adipic acid and N.sub.2 O. Thiemens and Trogler calculate that about 1 tool of N.sub.2 O per mole of adipic acid is formed as a side product in adipic acid processes. Assuming that 2.2.times.10.sup.9 kg of adipic acid are produced globally per year, about 1.5.times.10.sup.10 mol yr.sup.-1 of N.sub.2 O by-product or 10% of the annual output of atmospheric N.sub.2 O can be attributed to this single process.
M. Schiavello and coworkers (J. Chem Soc. Faraday Trans. 1, 71(8), 1642-8) studied various magnesium oxide-iron oxide and magnesium oxide-iron oxide-lithium oxide systems as N.sub.2 O decomposition catalysts. While magnesium oxide-iron oxide samples which were fired in air and which contained MgFe.sub.2 O.sub.4 demonstrated low activity, similar samples fired under reducing atmospheres and containing Fe.sup.2+ in solid solution demonstrated greater activity. The researchers calculated that Fe.sup.3+ ions in the ferrite phase are not catalytically active toward the subject reaction whereas Fe.sup.3+ ions contained in MgO together with Li.sup.+ are catalytically active when the ratio of lithium to iron is less than 1.
P. Porta and coworkers (J. Chem. Soc. Faraday Trans. 1, 74(7), 1595-603) studied the structure and catalytic activity of Co.sub.x Mg.sub.1-x Al.sub.2 O.sub.4 spinel solid solution for use as catalysts in decomposing N.sub.2 O into gaseous nitrogen and oxygen. The catalytic activity per cobalt ion in various N.sub.2 O decomposition catalysts was found to increase with increasing dilution in MgO. The distribution of cobalt ion among octahedral and tetrahedral sites in the spinel structure of Co.sub.x Mg.sub.1-x Al.sub.2 O.sub.4 was found to vary with temperature and the fraction of cobalt ions in octahedral sites was found to increase with increasing quenching temperature. The researchers concluded that catalytic activity generally increases as a greater amount of cobalt ions are incorporated into octahedral sites in the structure.
W. Reichle (Journal of Catalysis 94 (1985) 547) reported that various anionic clay minerals belonging to the pyroaurite-sjogrenite group, such as hydrotalcite (Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3.sup.2-).4H.sub.2 O can be thermally decomposed to form a product which is a useful catalyst for vapor-phase aldol condensations. Replacement of Mg by Fe, Co, Ni and Zn and/or replacement of Al by Fe and Cr also results in isomorphous double hydroxides which, on heat treatment, are rendered catalytically active. The reference also states that the activity of the catalyst is strongly affected by the temperature at which the hydrotalcite is activated.
U.S. Pat. No. 5,171,553, discloses a highly efficient, commercially viable process for removing N.sub.2 O from gaseous mixtures. The process utilizes catalysts comprising a crystalline zeolite which, at least in part, is composed of five membered rings having a structure type selected from the group consisting of BETA, MOR, MFI, MEL and FER wherein the crystalline zeolite has been at least partially ion-exchanged with a metal selected from the group consisting of copper, cobalt, rhodium, iridium, ruthenium and palladium.
Industry urgently desires to develop catalytic processes for destroying N.sub.2 O emissions prior to venting commercial process effluent streams into the atmosphere. Although catalytic decomposition of N.sub.2 O has been studied extensively in academic institutions, few commercially viable processes are known for decomposing N.sub.2 O into its respective components, namely gaseous nitrogen and gaseous oxygen, which utilizes a catalyst which exhibits the activity and life provided by the catalysts of the present invention.
U.S. Ser. No. 08/113,023 (the "'023 application"), filed Aug. 27, 1993, of which the present application is a continuation-in-part, teaches the use of various catalysts derived from anionic clay materials for decomposing N.sub.2 O in exhaust gas streams. The present invention is directed to certain improvements on the processes and catalysts taught by that application.
The '023 application is directed to a catalytic process for removing N.sub.2 O from exhaust streams utilizing a catalyst formed from one or more anionic clay minerals, which after appropriate heat activation, provide superior N.sub.2 O decomposition activity. The process comprises reacting an N.sub.2 O-containing stream in the presence of an effective amount of one or more of the enumerated catalysts under conditions sufficient to convert the N.sub.2 O to gaseous nitrogen and oxygen. Experimental results presented in Tables 1, 2 and 3 of the '023 application, which are the same as Tables 1, 2 and 3 of the present application, generally show excellent conversion of N.sub.2 O when applied to gas streams containing just N.sub.2 O and helium (see footnote a of each table). However, in test streams additionally containing 2% water (see footnote c of each table, and all test results marked with a "c") some of the catalysts provided poor N.sub.2 O decomposition (see Runs 3-7, 11, 13 and 18) while others still provided excellent results (see Runs 10 and 17). From these test results, it was seen that some factor in the catalysts which was affecting their ability to catalyze N.sub.2 O decomposition in water-containing gas streams. It is therefore desired to provide catalysts which are suitable for catalyzing N.sub.2 O-containing gas streams, regardless of whether or not such streams also contain water.